1. Field of the Invention
The present invention relates to key telephone systems and more particularly to a line circuit for use in key telephone systems, that includes means for detecting the presence of one or the other of two distinctively different ringing signals and in response to said detection provides at one or more associated key telephone subsets a visual signal distinctive only to the received signal.
2. Description of the Prior Art
Extensive use has been made in recent years of key telephone systems. Key telephone systems are customer controlled switching systems which conventionally provide access for a key telephone subset user to two or more telephone lines. Often several key telephone subsets are connected to a common group of two or more lines which are connectible to each of the key telephone subsets.
More recently key telephone systems have been employed in conjunction with large private automatic branch exchanges (PABX's) to provide a high degree of flexibility and convenience of telephoning service. For example in a large business many telephones may be provided as part of a private automatic branch exchange system, all of which may communicate with each other through an internally provided switching system which is operated by means of dial or push-button calling units associated with individual telephone subsets. Calls to and from an associated telephone central office are made and received through the locally provided switching equipment. In such a system it has been found expedient to include key telephone systems in addition to the private automatic branch exchange so that several telephone users in a common area or department may have access to a common group of lines in the branch exchange. In this manner each of the several users provided with key system service may make or answer telephone calls over any of the groups of lines dedicated to the key system.
A recent innovation in private automatic branch exchange service is the provision of inward calling. In inward calling each telephone line in the private automatic branch exchange may be dialed directly by central office subscribers through the private automatic branch exchange switching equipment, by the dialing of a number distinctive to the desired line.
When inward dialing service is provided the stations in the private automatic branch exchanges receive a distinctive ringing signal to indicate the presence of an inward dialed trunk call or a different distinctive signal to indicate the presence of a call originating from within the PABX. As long as the telephone subset connected to a PABX line in this arrangement is of the single line type, the local ringer included therein provides an audible indication of either of the distinctive tones received, so that the user can easily determine that the call originated locally or is an incoming central office call.
To date in PABX telephone systems with inward dialing it has been impossible to determine the nature of the incoming call at key telephone type subsets since typically such arrangements use a common signaling feature which prohibits the subset user from distinguishing a local intra-PABX call from an incoming trunk call. In such key telephone systems the line circuit conventionally converts all incoming ringings to a common visual and audible signal. The common audible signal once activated presents a single ring and typically a 60 impulse per minute signal to flash a lamp associated with the incoming line at each of the connected key telephone subsets. An example of a private automatic branch exchange useful for receiving inward dialed calls from a telephone central office as well as intra-PABX calls is the Type 311 Centrex PABX, manufactured by GTE Automatic Electric Incoporated. A key telephone system of the type which would be used with such a private automatic branch exchange is like that designated as a Type 10A2 Key Telephone System also manufactured by GTE Automatic Electric Incoporated. In the PABX referenced above a single ring of 2 second duration is provided for intra-PABX calls and two, 1/2 second rings separated by a 1/2 second period of silence, are provided on incoming trunk calls.
A suggested solution counted the one or two rings to provide identification of incoming calls. However since the PABX ringing singals are applied at random during the ringing cycle it would be possible to receive one ring of a two ring code and thus display an incorrect visual signal at a key telephone subset lamp. No other solutions are known to solve the problem presented in the system outlined above. Accordingly it is the object of the present invention to provide means for detecting the presence of either an intra-PABX call or an incoming trunk call received over a key telephone system line and provided a distinctive visual signal to indicate the characteristic of the incoming call.